Add cross testing with Zuul

Add the "zuul-client-zuul-functional" integration test.

This test ensures that the zuul-client CLI can interact
as intended with a "real" instance of Zuul.

Depends-On: https://review.opendev.org/752039

Change-Id: I23e3c4cc2721a000ec244696c27ab44c0a96d868

.zuul.yaml

@@ -1,3 +1,16 @@
+- job:
+    name: zuul-client-zuul-functional
+    parent: tox
+    description: |
+      Test that Zuul and zuul-client work together.
+    required-projects:
+      - zuul/zuul
+      - zuul/zuul-client
+    vars:
+      zuul_work_dir: "{{ zuul.projects['opendev.org/zuul/zuul'].src_dir }}"
+      tox_envlist: zuul_client
+    nodeset: ubuntu-bionic
+
 - project:
     vars:
       release_python: python3
@@ -13,6 +26,7 @@
         - tox-py38:
             nodeset: ubuntu-bionic
             timeout: 3600
+        - zuul-client-zuul-functional
         - build-python-release
     gate:
       jobs:
@@ -26,6 +40,7 @@
         - tox-py38:
             nodeset: ubuntu-bionic
             timeout: 3600
+        - zuul-client-zuul-functional
         - build-python-release
     promote:
       jobs: